It seems to me that there would be a lot more involved than just cutting a chamber. The .300 H&H has a much bigger head size, a belt, and is much longer. It would be simpler to rechamber to something like an Ackley Improved .30-06, and have fairly close to .300 H&H performance.

What kind of double rifle is it? Is it a collector's piece? Or is it one of the Russian/Turkish/whatever imports?

If you really want something traditional, have you considered a Ruger #1 Tropical in 375 H&H? This particular model in this particular chambering has a reputation for accuracy, and the #1 in this configuration superficially resembles the British Farquharson rifles (pictured above).

Been wanting a Ruger #1 in 375 H&H since I was a kid. But what I'll probably wind up with is a used #1 in 45-70 that I can put a long barrel on and restock to make a replica of an English Creedmoor long range rifle. But even that project has Winchester 1885 Single Shots lined up in front of it.

The rifle in the pic appears to be a Farquarson.....very nice...and it was bill Ruger's inspiration for the No. 1......one note on the 300 H&H....they had a reputaion for difficult extraction in some rifles because of the amount of taper in thier case walls....

The performance increase would be negligible going to a 300 H&H, but I would much rather have a double chambered for it than 30-06.

I love the old classic H&H cartridges. I have a M70 in 300, and a Whitworth in 375. Great old cartridges. Will do (and have done - many times over)everything the 'modern' SuperWildcatHowMuchPowderCanICramInIt UnbelievableSuperUltiMaxiMagnum will do.

If you handload, you can get some performance advantage out of the 300 H&H over the '06, but not much. That certainly wouldn't be my reason for doing it.

I like your idea, but as has been noted, there's a lot more than just running a reamer in it when you're looking at a double rifle. Pressure is the least of your concerns.

Problem with extraction was more a function of the cordolite powder spiking high pressure when the temps were HOT.. New powders today have pretty much eliminated that problem.. The long taper does promote forward brass flow..which weakens brass for reloading.. While a good old trooper..I'd keep the rifle in 30-06.. and maybe have Simmons make a set of barrels for something larger.. Like maybe 375 H&H..

Unless it's built very light I would think 300 HH would be doable, it's not all that powerfull but looks right for a double rifle. Look at some reloading manuals will give you an idea of the pressures, you could even load it down with some lead bullets and have fun with it.

Thanks for the replys..
The barrel is a Heym, and I am talking with a couple people as regards to the regulating of it to the H&H.....30-06, while readily available, and fairly cheap have doubled in price, and I thought I'd like to try the H&H as I have some already.

Correct in saying the long range accuracy wont help me in a DR, but bullet shape of the H&H seems appropriate.....